The invention relates to a switched capacitor (SC) filter with intrinsic anti-aliasing function and to an audio signal processor provided with such a filter.
The document GB 2 138 231 A describes a filter circuit comprising an SC filter that is adjustable with respect to its frequency response, in which in a neutral setting the effective audio signal path circumvents the SC filter, so that no decrease or increase of the amplitude of individual frequency ranges takes place. It is known furthermore from said GB 2 159 014 A to connect an anti-alias filter upstream of an SC filter.
With apparatus in the field of entertainment electronics, such as for example radio receivers, cassette recorders, CD players and the like, it is usual to provide in the audio signal path adjustable filter circuits through which the user can take influence on certain sound characteristics, such as for example treble boost and treble cut or bass emphasis and bass cut. The adjustment of suitable filter characteristics is made by the user by means of operating members provided therefor. The adjustable filter circuits, for purposes of influencing the audio signals in entertainment electronics equipment, are designed as a rule such that they permit in particular also a neutral setting in which they have as little influence as possible with respect to frequency and phase response.
In case the user desires no emphasis or de-emphasis at all of certain frequency ranges of the audio signal, he will select this neutral setting of the respective filter circuit.
With respect to numerous apparatus to be associated with the field of entertainment electronics, such as for example with compact radio receivers as used in particular for installation in motor vehicles, a trend can be observed for a long time to achieve a cost reduction in manufacture by putting together the entire electrical circuit of the receiver from as little as possible highly integrated electronic components.
FIG. 6 shows a block diagram of an exemplary audio receiver. The radio frequency signals delivered from a radio frequency antenna 10 are supplied to a radio frequency receiver, hereinafter referred to as xe2x80x9cradio frequency processorxe2x80x9d RFP. Radio frequency processor RFP comprises radio frequency pre-stages, tuners, intermediate frequency amplifiers and intermediate frequency filters as well as demodulators. A low frequency (LF) signal constituting the useful signal of the station received is fed to an audio signal processor ASP. This low frequency signal does not only include the demodulated audio signal, but also auxiliary signals for stereo reception, ARI identification, RDS and so on.
The audio signal processor ASP in particular converts the low frequency signal delivered by radio frequency processor RFP into audio signals which are fed to an audio power processor APP. The audio power processor APP comprises in particular a power amplifier 12 amplifying the output signals delivered from audio signal processor ASP, to such an extent that these can be reproduced through loudspeakers 13. Furthermore, the audio power processor APP comprises a voltage regulating module 14 for power supply.
The audio signal processor ASP may be fed, via additional analog inputs, with other audio signals, for example from cassette recorders, CD players or the like.
The audio receiver described is controlled in its entirety by a microcontroller 15 which is provided with an operating unit 20 that is accessible to the user. The exchange of data between microcontroller 15 on the one hand and radio frequency processor RFP, audio signal processor ASP and audio power processor APP on the other hand takes place through a conventional I2C bus.
FIG. 7 shows a schematic block diagram of the integrated audio signal processor ASP of the audio receiver shown in FIG. 6. The audio signal processor ASP has a number of analog audio signal inputs 50a to 50j that are connected to corresponding input lines of an analog multiplexer 55. A stereo output signal of analog multiplexer 55 is fed via output lines 57a and 57b to a chain of series-connected blocks 60, 62, 64, 66, 68 acting as signal processors.
The audio signals first enter a muting filter (xe2x80x9cmutexe2x80x9d) 60, and then in succession a volume control with loudness filter 62, a second muting arrangement with soft muting property (xe2x80x9csoft mutexe2x80x9d) 64, a bass control (xe2x80x9cbassxe2x80x9d) 66 as well as a treble control 68 (xe2x80x9ctreblexe2x80x9d). The stereo audio signals then are fed to a number of audio driver amplifiers 70, where they are conditioned to such an extent that they are suitable for controlling the power output stages in power amplifier 12 within audio power processor APP.
In this respect, each channel of the stereo signal is split to a first sub-channel for a first loudspeaker to be installed in the front of the passenger compartment and a second sub-channel for a second loudspeaker to be installed in the rear of the passenger compartment.
The audio signal processor comprises furthermore a number of usual functional blocks 72 to 82 which are necessary for processing the stereo differential signal, the ARI signal etc. Finally, there is provided a pause circuit 90 and a power supply means 95.
It is apparent to the expert that the audio signal processor ASP represented in FIGS. 6 and 7 does not constitute the sole possible embodiment of such a processor. In particular, the audio signal processor ASP may also be implemented without the functional units for processing FM stereo signals. The type and the number of the LF stages for taking influence on the audio signal may also vary for each particular case.
The circuit described hereinbefore involves the disadvantage that there is no complete integration of the audio filters 62, 66, 68 since numerous RC members with high resistances are realized by means of external components because of a too large space requirement of the resistors R.
When employing the so-called xe2x80x9cswitched capacitorxe2x80x9d filter technology (xe2x80x9cSC technologyxe2x80x9d), high resistances, which in case of direct implementation would cause consumption of much chip area, are substituted by arrangements with switched capacitors. Filter circuits with switched capacitors as such are known for example from xe2x80x9cAnalog MOS Integrated Circuits for Sigal Processingxe2x80x9d by Roubik Gregorian and Gabor C. Temes, John Wiley and Sons. This circuit technology facilitates indeed the complete integration of filter circuits on a semiconductor chip, but entails also other consequential problems.
In case of the xe2x80x9cswitched capacitorxe2x80x9d filter technology, the useful signals are sampled de facto with the frequency at which capacitors are switched that are used for re-storing charges. According to the basic sampling theorem of Shannon, an analog signal s(t) is described completely by equidistant sampling values in a time distance xcex94t only when the frequency spectrum of s(t) above an upper limit frequency fmax is identical to zero:                               f                      m            ⁢                          xe2x80x83                        ⁢            a            ⁢                          xe2x80x83                        ⁢            x                          =                  1                      2            ⁢            Δ            ⁢                          xe2x80x83                        ⁢            t                                              (        1        )            
The upper useful limit frequency thus is half of the sampling frequency, and signal portions of higher frequencies above fmax result in the so-called xe2x80x9caliasing effectxe2x80x9d, i.e., the energy content of these higher-frequency signal portions makes itself felt in the baseband in the form of interference signals. The principles of the aliasing effect are known, for example, from xe2x80x9cNachrichtentechnikxe2x80x9d, Vol. II: xe2x80x9cNachrichtenubertragungxe2x80x9d, by W. Rupprecht, Springer-Verlag, Berlin et al., 1982, pages 75 to 78.
A conventional countermeasure against this aliasing effect consists in providing in front of a filter chain realized in the form of xe2x80x9cswitched capacitorxe2x80x9d technology, an input low pass whose characteristics are suitably selected such that signal portions with frequencies higher than fmax are sufficiently strongly attenuated.
However, this circuit technology involves the disadvantage that background noise and distortions of the input low pass filter are superimposed on the useful signal also when the filter or filter chain is in the neutral setting.
It is thus an object of the invention to make available an adjustable SC filter with anti-alias function as well as audio signal processor provided with such a filter, which do not involve the prior art disadvantages and which in particular can easily be integrated completely and feature low noise and distortion also in the neutral position.
This object is met according to the invention by an SC filter according to claim 1. The subclaims indicate developments of the subject matters of the alternative independent claims.